In recent years, with increasing miniaturization and weight-saving of data terminals such as personal computers and cellular phones, more compactness and higher density wiring are coming to be required in the printed circuit boards installed in these devices. In order to satisfy these needs, it is necessary to make the wiring width narrower and to make the gaps between the wiring closer. To match the higher processing speeds of electronic devices, it is also required to increase the number of inputs and outputs with which these electronic devices are provided.
To meet these demands, the mounting of elements on printed circuit boards changed from the pin intercalation type to the surface-mounted type, and then to the area array type represented by a BGA (ball grid array) which uses a plastic substrate. In a substrate on which a bear chip such as a BGA is mounted directly, the chip connections are often made by wire bonding using thermal supersonic waves with application of pressure. It is also required to increase the number of terminals connected by this wire bonding or to make the terminals narrower.
In the prior art, the substrate of these printed circuit boards consisted of a laminate obtained by superimposing a predetermined number of prepregs or the like having a matrix of an electrically insulating resin, and forming them into one piece by the action of heat and pressure. The electrically insulating resin was generally a thermosetting resin such as a phenol resin, epoxy resin, polyimide resin or bis-maleimide-triazine resin. A thermoplastic resin such as a fluororesin or polyphenylene ether resin may also be used.
As the method of forming the conductor circuit on the printed circuit board, the subtractive method is widely used. In this method, a conducting foil such as a metal foil is laminated on a surface (one side or both sides) of the aforesaid laminate, and heat and pressure are applied to form a one-piece conductor-clad laminated sheet. A circuit pattern is then formed by removing the conductor foil on this conductor-clad laminated sheet by etching.
In this conductor-clad laminated sheet, to prevent peeling of the circuit pattern, it is desirable that the conductor foil and insulator layer comprising the laminate of prepregs are firmly stuck together. For this purpose, in the prior art, the adhesive force between the conductor foil and insulator layer was increased by roughening the surface of the foil so as to produce an anchor effect of the resin in the insulator layer as disclosed in JP-A No. 04-211941 or “High-Density Circuit Boards” (by Electronic Material Group Editorial Dept., Kogyo Chosakai, 20 May, 1986, p. 149-157).